Proposed FITS Standard Multiple World Coordinate Systems

WCSTools 2.9 implements the use of multiple world coordinate systems, with
the WCS to which a keyword applies designated by a suffixed upper-case alphabetic
character.

Thus CTYPE1 applies to the default WCS, which may be named using
WCSNAME. A second WCS could use keywords such as CTYPE1A
and WCSNAMEA. Up to 27 (no suffix, A-Z) different sets of WCS keywords
may be specified.

In the WCSTools programs, the WCS to be used is specified by adding
%WCSNAME to the end of a filename specified on the command line.
This is added after the extension of a multi-extension FITS image is specified:

fitsfile,extension_name_or_number%WCS_name_or_letter

imwcs always sets the default (no suffix) keywords.

Sometimes getting from image pixels to world coordinates is best though of a
two step process, pixel<->focal plane<->sky, for example.
The keyword WCSDEPx, indicating a named WCS conversion which must
be applied to pixel coordinates before WCS conversion x can be applied, could
be added to indicate the name or letter of the requisite WCS.
This convention is described by Doug Mink and Steve Allen in a memo,
"The WCSDEP Convention", available in
PDF or
Postscript format.